Probably because it's made from used milk jugs, which are being generated all the time, and its manufacturer isn't as mercenary as most (this is all just guesses). Given what it is made from the Orcaboard ought to be cheaper, and I would reward the company for their honest pricing by buying it if I were in your shoes. I don't know about 50 years, but I bet it will last as long or longer than the Frame it All.

Here is a link that might be useful: A comic amusingly related to the topic

IMO, the solid composites don't feel much like real wood while the hollow ones do, but for your application, that doesn't matter. The only questions I have are if either or both list UV resistance as part of their feature set, or if they have restrictions against ground contact.

I don't know about orcaboard, but I am familiar with composite decking materials like Trex and Choicedek. They are a mixture of plastics and wood fibers, and are absolutely great for decks, since they don't rot.

However, they have little strength, much less than dimensional lumber, so you need to space deck supports closer together to support them.

There might be a clever way to reinforce them. But I'd be concerned about the strength issue, even for a 4X4 bed.